Unlike distilled water, de-ionized water wont be free of organic materials like algae, so biocide is needed if you use it as coolant.
Distilled removes bacterial contaminants as well, de-ionised doesn't generally - that's not to say that you can use distilled without biocide, you will end up contaminating the water one way or another.Sorry but that's wrong, the only difference between DI and distilled is the process used to create them, their both de-mineralized/purified water it's just they have been created via different methods (DI's being easier hence why it has taken over from distilled in supply).
However, deionization does not significantly remove uncharged organic molecules, viruses or bacteria, except by incidental trapping in the resin
You are much better off looking for RO water. Try your local aquarum place, couple of quid gets you much more than 1L.